In recent years, high-strength cables are required in various fields. As one of the high-strength cables, for example, a tether cable that is used in a sea or water is known.
In an unmanned exploration system that is used in the sea or the water, a tether cable is used to transmit electrical power and an optical signal. FIG. 6 illustrates a sectional structure of a tether cable. A tether cable 1000 has plural power lines 1001, plural optical fiber units 1002, reinforcing elements 1003 and 1004, and an outer coating 1005. The reinforcing elements 1003 and 1004 constitute a twisted layer, and can be resistant against tension in a longitudinal direction and against a force applied in a torsional direction and prevent extension of the tether cable 1000 to prevent the tether cable 1000 from being bent.
The reinforcing elements 1003 and 1004 of the tether cable 1000 have plural fiber elements disposed in a coating element, and each of the fiber elements is made of a PPTA (polyparaphenylene terephthalamide) corresponding to an aramid fiber. As the PPTA, Kevlar (registered trademark) that is manufactured by Du Pont-Toray Co., Ltd. and TOWARON (registered trademark) that is manufactured by Teijin Aramid are used. As such, the reason why the aramid fiber is used as the fiber element is that, if the cable is elongated, because the cable becomes weight and is fractured due to its weight, the aramid fiber is used to decrease the weight of the cable and obtain high strength. The reinforcing elements 1003 and 1004 are twisted in two layers and opposite directions, and are solidified with a resin in the outer coating (for example, refer to Patent Document 1).
Patent Document: Japanese Patent Application Publication (JP-B) No. 7-54651